Binary single crystal relaxor ferroelectrics such as Pb(Mg1/3 Nb2/3)1-x TixO3 (PMN-PT) have shown great promise for broad bandwidth transducers. Such a binary single crystal has an elastic compliance that is about 4.5 times greater than that of PbZrO3—PbTiO3 (PZT-4), a traditional piezoelectric ceramic of which Type I PZT is commonly used in a broad range of applications. The material also has a piezoelectric coefficient d33 that is 6.5 times that of Type I PZT-4 and an electromechanical coupling coefficient k33 that is greater than about 90%, whereas k33 for Type I PZT is less than about 70%. The improved elastic compliance allows the PMN-PT to be provided at a dramatically reduced element size for a predetermined resonant frequency. The improved piezoelectric coefficient allows the smaller element to maintain acoustic intensity. The improved coupling coefficient provides a larger operating bandwidth, which is important for a power delivery system. In addition, the improved coupling coefficient k33 provides high receive sensitivity further above the fundamental resonance frequency than is the case with a ceramic.
However, the use of a binary single crystal relaxor such as PMN-PT is limited due to its low Curie temperature, Tc, and morphotropic behavior, that is, the phase transition from rhombohedral to tetragonal at a phase transition temperature Trt. The Tc is an important parameter because the class of materials that includes the single crystal relaxor ferroelectrics does not recover once the single crystal relaxor ferroelectrics Tc is exceeded without application of a large electric field to re-polarize the crystal. The vibration characteristics of the material are partially lost once the Trt is exceeded, and these characteristics do not recover if the temperature is lowered. For a material used in ultrasonic applications, these vibration characteristics are a critical property, and the Trt limits the maximum use temperature of the crystal. The dielectric constant and piezoelectric coefficient of PMN-PT are also highly temperature dependent. For example, PMN-33% PT has a 75% change in dielectric constant in the temperature range of 0-50° C. (32-122° F.). This change adversely affects transducer impedance and matching circuitry, which in turn affects the power delivery system. Thus, a dielectric constant that does not change significantly with temperature is important for reliable operation such a system.
For applications in which space is an issue, such as sonar applications, such temperature dependence affects performance. The power required to drive the electronic circuitry continues to increase with complexity of the circuits, which further increases the operating temperatures of the crystals and all of the equipment associated with such applications in such confined spaces. PMN-PT also has a coercive field which is six times lower than Type I PZT ceramic, so that an electrical bias has to be applied to keep the crystal from depoling during high driving, bipolar applications.
Efforts have been made to overcome the disadvantages of PMN-PT. New binary crystals that possess higher Curie temperatures have been developed such as Pb(Sc1/2Nb1/2)O3—PbTiO3 (PSN-PT), Pb(Sc1/2Ta1/2)O3—PbTiO3 (PST-PT), Pb(Yb1/2Nb1/2)O3—PbTiO3 (PYN-PT), Pb(In1/2Nb1/2)O3—PbTiO3 (PIN-PT), Pb(Co1/2Nb2/3)O3—PbTiO3 (PCN-PT) and Pb(Co1/2W1/2)O3—PbTiO3 (PCW). Each has a relatively high Tc near their morphotropic phase boundary compositions. However, their crystal growth is limited due to the instability of the perovskite phase in the melts of these materials. Thus, it is difficult to grow single crystals from melts of these compositions, due to the slow growth rates and instability. While these binary materials have a promising Tc, in the range of 260-360° C. (500-680° F.) and a phase transition temperature of 50-160° C. (122-320° F.), small sized, polycrystalline crystal grains typically result due to this instability. These polycrystalline grains are not practical since they do not result in single crystals of sufficient size which can be produced at a reasonable cost, if they can be produced at all.
Increasing the temperature usage range of the PMN-PT has also been attempted by developing relaxor-PMN-PT ternary systems such as Pb(Sc1/2Nb1/2)O3—Pb(Mg1/3Nb2/3)O3—PbTiO3, (PSN-PMN-PT), Pb(Yb1/2Nb1/2)O3—Pb(Mg1/3Nb2/3)O3—PbTiO3 (PYN-PMN-PT), Pb(In1/2Nb1/2)O3—Pb(Mg1/3Nb2/3)O3—PbTiO3 (PIN-PMN-PT) and Pb(Mg1/3Nb2/3)O3—PbZrO3—PbTiO3 (PMN-PZ-PT). In the ternary PIN-PMN-PT, higher mole percentages of PIN produce improved Trt and an improved coercive field, Ec, as compared to binary PMN-PT crystals, while other dielectric properties and piezoelectric properties remain similar to PMN-PT crystals. However, one of the drawbacks with such ternaries has been a limitation on the PIN concentration, as no concentrations higher than 28 mole % have been grown by the Vertical Bridgeman method due to the difficulty in preventing the formation of secondary phases, in particular, the pyrochlore phase during the crystal growth process.
What is needed are single crystal ternary materials that can be conventionally grown for use as transducers that are capable of use at higher temperatures. These crystals must have a higher Curie temperature Tc, so that they can be driven at higher powers in higher temperature environments. In addition, these materials should have a dielectric constant that is as flat as possible across the temperature range of operation. It is also desirable that the single crystal materials of the present invention do not have a Trt that is within the operating range.